Give BT a call and they will send out a returns bag free of charge
phone CS 03301234150 and request bag
As @imjolly has it above 03301234150
Select Billing when you call 0330.1234.150.
Do keep a Proof of Posting when you send equipment back to BT. They are hopeless at recording equipment returned and will probably ask you to pay later as you haven't returned it!
Always get proof so that you can quote the number on the receipt later.
Richard